This invention relates to optical fibre sensing systems and relates more specifically to optical fibre coil assemblies for incorporation in such systems as sensors or other component parts (e.g. reference or balancing coils).
In many coherent optical fibre sensing systems sensor and reference or balancing optical fibre coils of approximately the same length are provided. The sensor coil needs to have a greater sensitivity than the reference or balancing coil, a sensitivity ratio of 10:1 probably being adequate.
In the case of interferometer type arrangements a balance (delay) coil and sensor coil are placed in the respective arms of the interferometer. The sensor coil is required to be sensitive to the measurand (e.g. acoustic pressure in the case of hydrophones) and the sensor and balance coils co-operate to maintain coherence of light signals applied to the interferometer and to reduce the phase noise generated after combining the light outputs from the arms of the interferometer.
In the case of a heterodyne optical sensing system of the above kind, the respective lengths of the coils may be adjusted to provide a predetermined path length difference whilst the frequency of light signals applied to the interferometer is ramped. In this way a heterodyne frequency on which the sensed signals are phase modulated is provided at the usual receiver photodiode.
In time-multiplexed pulsed heterodyne optical fibre sensor systems in which, for example, two pulses of slightly different frequencies and of predetermined duration and time relationship are transmitted along a sensor optical fibre having partially reflective discontinuities therealong from which small proportions of the light pulses are reflected back along the fibre at each fibre discontinuity, the signals reflected back from the successive discontinuities are caused to interfere with the signals reflected from the preceding discontinuities in order to produce heterodyning which provides a detectible electrical beat signal the modulation of which is dependant upon changes in length of the optical fibre sensor elements between the discontinuities. In such heterodyne systems there is a need for optical fibre reference coils which are insensitive to the measurand and to other environmental influences which serve to reduce the optical phase noise. This may be achieved as described in our Patent No. GB 2184237, by providing a reference sensor having low sensitivity to the measurand in series with the usual sensors or sensor elements. The reflected output signal from the reference sensor is assumed to be mainly noise from the laser light source for the system or from the environment which is common to all of the sensors or sensor elements of the system. The reflected output from the reference sensor may therefore be subtracted electrically from the other reflected outputs from the sensors or sensor elements in order to cancel the common mode noise.
In another system a reference coil is introduced to minimise a path length imbalance in an optical circuit used for phase detection in order to reduce the common mode phase noise from the laser light source of the system or from environmental sources.
In all the above described arrangements, the reference or balance coil is usually made insensitive by isolation from the measurement. This is reasonably easy at mid acoustic frequencies (above about 200 Hz), but becomes progressively more difficult at lower frequencies. The structures required to mount the coil can become bulky, fragile and not very efficient.
One aspect of the present invention is directed to optical fibre coil assembly constructions, especially applicable to optical fibre hydrophones, in which a reference or balance coil needs to be protected from the influence of air or water-borne acoustic pressures, but it is also applicable to other optical fibre sensing systems in which insensitivity to pressure or mechanical vibrations is required.